1. Field of the Invention
This invention relates to the detection of fingerprints and the like, and in particular to the enhancement of polycyanoacrylate-developed fingerprints on human skin and other surfaces.
The skin is covered with a constantly changing film of sweat as the glands continuously secrete fluid, therefore any substance that is deposited on the surface of the skin is likely to be diluted and become blurred.
The sweat layer is a mixture of secretions from mainly eccrine and sebaceous glands (eccrine secretions; amino acids, urea, chlorides, and sugars; sebaceous secretions; fatty acids, glycerides, assorted hydrocarbons). Only eccrine glands are located on the palmar regions, and materials secreted from the sebaceous glands are transferred to the hands by touching areas such as the face and the back. (Detection of Fingerprints on Skin, D. S. Allman and C. A. Pounds, For. Sci. Rev. 3(2) 1991) Therefore it is possible that the materials deposited in a latent print may differ qualitatively and quantitatively from those of the skin surface on which the print is deposited.
After death the glands no longer secrete material and thus the dilution process ceases. The surface temperature will fall from that of live skin (near 32.degree. C.) to ambient over a few hours. During this time the waxes and oils of the print will not be able to maintain their liquid state. However, before ambient temperature is reached diffusion may still occur on the postmortem surface leading to deterioration of ridge detail. Due to the absence of good detection methods for latents on skin this problem has not been well studied.
2. Description of the Prior Art
A number of techniques for the detection of fingerprints have been documented.
Fingerprints developed by cyanoacrylate ester (Super Glue) fumes on surfaces such as metals and plastic are well recorded in the literature. More specifically, the fingerprints develop a white residue on their ridges which is often visible to the naked eye as the cyanoacrylate ester molecules polymerize. This polymeric film provides a protective layer which helps preserve the ridge detail. The prints are easily seen on transparent surfaces. But on opaque surfaces (especially white ones) contrast may De poor. This makes it difficult to interpret print detail. Accordingly, dusting with powder or fluorescent dye staining is often used to enhance the print image.
There are many fluorescent dyes available for staining polycyanoacrylate ester such as Ardrox, Brilliant Yellow, Rhodamine-6-G and DCM. The dyes are applied as a methanol solution which is adsorbed onto the surface of the print as well as onto the background. Washing with fresh solvent to reduce the dye in the background also reduces the fluorescence intensity of the print. Fluorescence Is excited by a variety of high power light sources including the xenon arc lamp, Ar ion, Cu vapour or frequency-doubled Nd:YAG lasers. These are expensive high power light sources in the price range of $10-20,000 for lamps or over $100,000 for lasers. Such costly equipment restricts the availability of this technology to police forces which have large budgets.
A further disadvantage is the limitations of these techniques when applied to the detection of fingerprints on human skin. It cannot be overemphasized how important it would be to police authorities to have a technique suitable for the reliable detection of identifiable latent prints on murder victims.
Probably the most commonly used method for detecting prints on cadavers is the iodine-silver plate transfer technique, described for example in J. For. Sci. 22, 599 (1977) J. M. Adcock. The procedure involves fuming of the suspect area with iodine vapour which causes discoloration of the print. This is followed by the application of a polished silver plate to the print. Strong light is used to darken the silver iodide produced which can then be recorded by photography.
Powder techniques that have been applied to the recovery of prints on murder victims include the Magnabrush technique and the Kromekote.RTM. Lift technique. The methods rely on lifting of the print from the skin surface before the print can be identified. A significant level of print. detail will automatically be lost during the lifting process and this is one of the major disadvantages of such techniques.
Recently reports have appeared detailing the development of latent prints on human skin by cyanoacrylate fuming in a "tent" over the body followed by Rhodamine-6-G as a staining agent. However, the background fluorescence resulting from absorption of dye into the skin considerably reduces the contrast between the print and the background leading to loss of detail.
Further, in Fluorescent Metal-Ruhemann's Purple Coordination Compounds: Applications to Latent Fingerprint Detection, Journal of Forensic Sciences, JFSCA, Vol. 35, No. 1, Jan. 1990, pp. 25-34, E. R. Menzel et. al., latents were developed on paper with ninhydrin which reacts with the amino acids to give the dye Ruhemann's purple. This dye was then used to chelate europium ions thus becoming weakly fluorescent under laser illumination. The efficiency of this method is very low, and expensive time-resolved imaging is required to minimize the background fluorescence.